1. Field of the Invention
The present invention relates to a polygonal mirror for deflecting and scanning a light beam in an image exposure device, the polygonal mirror having a plurality of reflecting flat mirror facets disposed on the outer surface of a polygonal body which is rotatable about its central axis.
2. Prior Art
Image exposure devices used in digital image forming apparatus such as digital copying machines, facsimile machines, and laser printers, for example, convert an information signal to be recorded into a modulated light beam, which is deflected by a deflector to scan the surface of a photosensitive medium.
One example of such a deflector or scanner is a polygonal mirror which is rotatable about its own axis. The polygonal mirror has a plurality of reflecting flat mirror facets disposed on the outer circumferential surface of a polygonal body which is rotatable about the axis. When the polygonal body is rotated about the axis at a constant speed, the mirror facets are moved along a constant circumferential path to deflect the modulated light beam for thereby scanning the photosensitive medium.
Modern image forming apparatuses operate to produce images at higher densities and at higher speeds. Therefore, the polygonal mirrors used therein as deflectors or scanners are larger in size and higher in speed. One problem with the larger and higher polygonal mirrors is that they are apt to be distorted when rotating at high speeds.
FIG. 11 of the accompanying drawings shows the relationship between the printout copy count (indicated on the horizontal axis) of an image forming apparatus and the rotational speed (indicated on the vertical axis) of a polygonal mirror used in the image forming apparatus, the polygonal mirror having an inscribed circle whose diameter is 22 mm and six mirror facets. When a larger polygonal mirror is rotated at a higher speed, it is distorted into an irregular shape making the mirror facets uneven. This causes the problems that no optimum beam spot diameter can be maintained by the deflected light beam and no optimum beam intensity distribution can be achieved by the deflected light beam. More specifically, the polygonal body of the polygonal mirror has ununiform moments of inertia about the axis of rotation, and tend to be deformed irregularly about the axis upon rotation thereof. As shown in FIG. 12, the polygonal mirror which is being rotated is distorted such that the angular corners thereof are forcibly projected further radially outwardly as indicated by the broken line.